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# Berry aneurysm and subarachnoid hemorrhage

- Lesion

A berry aneurysm is a thin pouch at a fork of the brain's arteries; when it bursts, blood fills the space around the brain.

Updated Nov 12, 2026

## Key points

- About 90 percent arise in the anterior circulation: anterior communicating artery junction most often, then posterior communicating and middle cerebral bifurcation.
- Wall: no muscular media and a fragmented internal elastic lamina; the sac is made of thickened hyalinised intima and adventitia.
- Risk factors: smoking, hypertension, autosomal dominant polycystic kidney disease, Ehlers-Danlos type IV, family history; they are acquired, not congenital, despite the old name.
- A posterior communicating aneurysm can compress the third nerve: a dilated pupil with ptosis.
- After rupture: rebleeding in the first days, vasospasm with delayed ischemia around days 3 to 14, and hydrocephalus from blocked CSF absorption.

## What it is

Saccular aneurysms are the commonest cause of nontraumatic subarachnoid hemorrhage. They form at branch points of the circle of Willis, where the muscular media is naturally deficient, and grow over years. Rupture floods the subarachnoid space with blood under arterial pressure, causing the ‘worst headache of life’, and is followed by dangerous vasospasm and hydrocephalus. Other aneurysm types (mycotic, fusiform, traumatic) are less common.

## Step by step

- Hemodynamic stress acts on a branch point where the media is thin.
- The wall degenerates, the elastic lamina fragments and a pouch bulges out.
- Smoking and hypertension speed growth; a larger sac is more likely to rupture.
- Rupture, often with straining, pours blood into the subarachnoid space.
- Blood breakdown products trigger arterial spasm and delayed infarction.
- Organising blood in the meninges blocks CSF absorption and causes hydrocephalus.

## Intracranial aneurysm types

| Type | Shape and site | Cause |
| --- | --- | --- |
| [Saccular (berry)](https://durgical.com/en/conditions/subarachnoid-haemorrhage/) | Round pouch at branch points of the circle of Willis | Acquired degeneration at a site with thin media |
| Fusiform | Spindle-shaped dilatation, often basilar | Atherosclerosis |
| Mycotic (infectious) | Distal branches | Septic emboli, usually from infective endocarditis |
| Traumatic or dissecting | Any artery | Trauma, arterial dissection |

## Clinical notes

- Sudden severe headache with neck stiffness and vomiting; CT shows blood in the basal cisterns, and lumbar puncture shows xanthochromia if CT is negative.
- Screening relatives and patients with polycystic kidney disease is considered when there is a family history.

## Memory aid

Berries hang at forks: the front fork (anterior communicating) carries the most, and the side fork (posterior communicating) can squeeze the third nerve.

## Test yourself

Think of the answer, then tap the question to see it.

## What is missing from the wall of a saccular aneurysm?

The muscular media; the internal elastic lamina is fragmented and the sac is made of intima and adventitia.

## Which kidney disease is associated with berry aneurysms?

Autosomal dominant polycystic kidney disease.

## When is vasospasm most likely after aneurysmal subarachnoid hemorrhage?

Roughly from day three to day fourteen, causing delayed ischemic deficits.

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## Elsewhere in DurgiPedia

- [Meningeal syndrome](https://durgical.com/en/clinical-exam/meningeal-syndrome/)

## Connections

## Related 5

- [Small vessel disease: hypertensive and amyloid hemorrhage](https://durgical.com/en/general-pathology/small-vessel-disease-hypertensive-and-amyloid-hemorrhage/)
- [Vascular malformations of the brain](https://durgical.com/en/general-pathology/vascular-malformations-of-the-brain/)
- [Hemorrhage: petechiae, purpura, ecchymosis, hematoma, and what blood loss does](https://durgical.com/en/general-pathology/hemorrhage-petechiae-purpura-ecchymosis-hematoma-and-what-blood-loss-does/)
- [Aneurysms: types and pathogenesis](https://durgical.com/en/general-pathology/aneurysms-types-and-pathogenesis/)
- [Autosomal dominant polycystic kidney disease](https://durgical.com/en/general-pathology/autosomal-dominant-polycystic-kidney-disease/)

## Related conditions 6

- [Subarachnoid haemorrhage](https://durgical.com/en/conditions/subarachnoid-haemorrhage/)
- [Autosomal dominant polycystic kidney disease](https://durgical.com/en/conditions/autosomal-dominant-polycystic-kidney-disease/)
- [Ehlers–Danlos syndrome](https://durgical.com/en/conditions/ehlers-danlos-syndrome/)
- [Arterial hypertension](https://durgical.com/en/conditions/arterial-hypertension/)
- [Oculomotor (third) nerve palsy](https://durgical.com/en/conditions/oculomotor-nerve-palsy/)
- [Hydrocephalus](https://durgical.com/en/conditions/hydrocephalus/)

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: [Subarachnoid Hemorrhage](https://www.ncbi.nlm.nih.gov/books/NBK441958/) (StatPearls (NCBI Bookshelf), 2026); [Autosomal Dominant Polycystic Kidney Disease](https://www.ncbi.nlm.nih.gov/books/NBK532934/) (StatPearls (NCBI Bookshelf), 2026).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/general-pathology/berry-aneurysm-and-subarachnoid-hemorrhage/